Fip belly

Really quick run down...
Had a cat pass away from dry fip earlier this year. Few month later an approximate 9 month old cat starts living in our yard. Once we bring him in get him tested and treated... Seems healthy. That was in Start of June. Since June he has gained 5 pounds... All in his belly. He has been treated for tapeworms and tested negative for other parasites. We battled an eye infection and I notice his third eyelid is always a little out since. The cats belly is really very big and the rest of him still pretty little. He doesn't do much playing or activity and his skin rolls when you pet him. (Which our last cat did too). How can I tell if he's overweight or sick? Vet said he's not sure which it is either but to start him on a diet. I don't want to make him diet if he's sick. He still has an appetite. Any guidance would be helpful thanks is that a typical amount of weight to gain for a rescue in 4.5 months?

Well, it's hard to say...depending on how underweight he was in the beginning, BUT I wouldn't think he would be gaining all of the weight in his tummy.  He should be gaining equally all over

I know you can't test for FIP, but has your Vet done any other testing to try to figure out what's wrong, because it seems something is clearly wrong for a 9 month old to not be playful and always have his third eyelid out.

Hi there. The vet did the fecal exam which came back negative and he mentioned the belly tap to check if it's fluids. Not sure what other testing we should try? I know that if it is fip it'll progress pretty quickly so part of me doesn't want to put a needle in his belly since it won't change the outcome but does anyone know if the needle in the belly hurts them a lot?
I've gotten some low calorie food and some new toys to try to see if he responds. I would say he maybe has gotten just a little bigger in other places but it's certainly noticeable that his belly is much too big for his frame as he is still little. Anyway thanks... I think it's a waiting game to see but does feel good to get some support on here sometimes. That's what helped me with my last one so I appreciate it. I'm thinking about re treating for worms despite negative fecal test just in case.

Well, you're right in that if it IS FIP, the end result won't change.  And even the "belly tap" won't tell you that is IS FIP, although if he's just fat, it might let you know that, and then your Vet could start looking for the reason for his third eyelid showing.  

Honestly, I don't know if it hurts to have that belly tap done or not.  I don't really think so, because I've watch them get a urine sample by doing a needle into their bladder before, and my cat didn't even seem to notice.  You could call your Vet and ask.  But if you choose not to do that, then just give him plenty of love, and hopefully he'll just grow into his belly.   It would be horrible to have to go thru FIP again

That's actually really good to know that you've experienced that before because I go back and forth between just wanting to know and not doing it. So that is helpful information thank you. Yeah I think for now I'll just give love love love and see if it gets worse. Thanks for the support